A trench-gate type power MOSFET is a power transistor of a vertical structure having a channel formed in a thickness direction of a semiconductor substrate by providing a gate electrode in a trench grooved in a direction crossing a main surface of the semiconductor substrate through a gate insulating layer, providing a source region on a front layer portion of the main surface of the semiconductor substrate, and providing a drain region on a back surface side opposite to the main surface of the semiconductor substrate.
U.S. Pat. No. 5,998,833 (Patent Document 1), Japanese Patent Application Laid-Open Publication No. S63-296282 (Patent Document 2), and Japanese Patent Application Laid-Open Publication No. 2006-202931 (Patent Document 3) disclose a technique regarding the trench-gate type power MOSFET.